1. Field of the Invention
The present invention relates to a wire dot print head for use in an impact printer.
2. Description of the Related Art
There is a prior art impact printer on which a wire dot print head is mounted for performing printing by print wires. According to this wire dot print head, print wires are fixed to tip portions of armatures each attached to a free end of a flat spring. The armatures are attracted to cores owing to magnetic flux of a permanent magnet. Coils forming electromagnets confronting the armatures are energized during printing so as to cancel the magnetic flux of the permanent magnet for releasing the armatures so that each print wire is protruded. A platen is disposed to confront the print head, and the protruded print wire strikes against the platen by way of an ink ribbon and a print medium, so that printing is performed.
The platen is normally cylindrical so as to smoothly feed the print medium. The print head performs printing while moving in an axial direction of the platen. The print wires are arranged in two columns in the direction to cross at right angles with the axial direction of the platen at a tip portion of the print head (through which each print wire protrudes) confronting the platen. The print wires positioned at both ends of two arrays (hereinafter referred to as end print wires) and those positioned at central portions of two arrays (hereinafter referred to as central print wires) are differentiated in strokes extending to the platen. Each stroke extending from the end print wire to the platen is longer than that extending from the central print wire to the platen. Accordingly, the end and central print wires are differentiated in time needed for reaching the platen during printing, wherein it takes longer time for the end print wire to reach the platen compared with the central print wire to reach the platen. Therefore, in case of driving the end print wire, a driving time is set to be longer than that of the central print wire so as to adjust timing, which leads to a great loss of power.
Since the end print wire is slower than the central print wire in time needed for returning to an original position from the platen, the print head may start moving before the end print wire completely returns to the original position during printing at high speed, the end print wire is liable to be caught by an ink ribbon. Furthermore, since the end print wire needs to be driven continuously when an underline is printed, printing speed needs to be reduced, which causes a problem of reduction of printing speed as a whole.